1. Field of the Invention
The present invention relates to compression release mechanisms for small internal combustion engines of the type used in a variety of applications, such as lawnmowers, generators, pumps, tillers, pressure washers and other lawn and garden implements, or in small utility vehicles such as riding lawnmowers, lawn tractors, and the like.
2. Description of the Related Art
Generally, the intake and exhaust valves of small internal combustion engines may be actuated directly by a camshaft located in the cylinder head, or may be actuated indirectly through the use of rocker arms, tappets, or other similar means. For example, in many existing L-head and overhead valve (xe2x80x9cOHVxe2x80x9d) engines, the crankshaft drives a camshaft which is located within the crankcase and is disposed parallel to the crankshaft, and lobes on the camshaft actuate lifters, push rods and/or rocker arms to open and close the valves. In overhead cam (xe2x80x9cOHCxe2x80x9d), engines, a camshaft located in the cylinder head of the engine is driven from the crankshaft, and includes lobes thereon which directly actuate intake and exhaust valves. One such overhead cam engine is disclosed in U.S. Pat. No. 6,295,959, assigned to the assignee of the present invention, the disclosure of which is expressly incorporated herein by reference.
At engine cranking speeds during engine starting, the intake and exhaust valves are both closed as the piston rises toward its top dead center position, and substantial pressure is built up in the combustion chamber which resists movement of the piston toward the top dead center position. This pressure must be overcome to crank the engine for starting, and typically requires a substantial amount of force to be exerted by the operator, such as by pulling on the rope of a recoil starter. Therefore, small internal combustion engines typically include a type of compression release mechanism to aid in engine starting.
Compression release mechanisms for small internal combustion engines are usually operable at cranking speeds to prevent the exhaust valve from fully closing as the piston reaches its top dead center position, thereby allowing venting of pressure from the combustion chamber. In this manner, cranking of the engine is much easier and requires less force to be exerted by the operator. When the engine reaches a predetermined speed after starting, the compression release mechanism is automatically rendered inoperative, such that the exhaust valve fully seats or closes as the piston approaches its top dead center position to allow combustion to proceed in a conventional manner.
A problem with many known compression release mechanisms is that such devices include a large number of individual parts, and are often mechanically complex. Further, such devices typically take up an undesirably large amount of space around the camshaft of the engine.
What is needed is a compression release mechanism for small internal combustion engines which includes a relatively few number of parts, is durable, and which is compact in construction.
The present invention provides a compression release mechanism for small internal combustion engines, including a compression release member having an auxiliary cam and a weight section. The compression release member is supported for rotation on an annular bearing surface which is in eccentric relation to the longitudinal axis of the engine camshaft. At engine cranking speeds, the compression release member rotates with the camshaft, and the auxiliary cam projects beyond the base circle of a cam lobe on the camshaft to periodically engage a valve to vent pressure from the engine combustion chamber during the compression stroke of the piston to aid in cranking the engine during starting. After the engine starts, rapid rotation of the camshaft causes the compression release member to rotate under the inertial load of the weight section thereof to a position in which the auxiliary cam is retracted within the base circle of the cam lobe such that combustion may proceed in a conventional manner.
More specifically, in one embodiment, the compression release mechanism includes a first collar which is rotationally fixed with respect to the camshaft, the first collar having a hub portion eccentric with respect to the camshaft. A second collar includes an auxiliary cam and a weight section, and is supported for rotation on the hub portion of the first collar between first and second positions. In the first position, which corresponds to engine cranking speeds, a spring connected between the first and second collars biases the second collar to a rotational position in which the auxiliary cam projects beyond the base circle of the cam lobe to engage and at least partially open the valve. In this manner, a portion of the pressure within the combustion chamber is vented during the compression stroke of the piston to aid in engine cranking. After the engine is started and the rotational speed of the camshaft rapidly increases, the inertial load of the weight section of the second collar overcomes the bias force of the spring, and the second collar rotates to a rotational position in which the auxiliary cam is positioned within the base circle of the cam lobe such that the auxiliary cam does not engage the valve, allowing combustion to proceed in a conventional manner.
Advantageously, the construction of the compression release mechanism, which includes the first collar positioned on a crankshaft adjacent the cam lobe, and the second collar supported for rotation upon the first collar, it is very compact in construction, such that the compression release mechanism takes up a minimal amount of space around the camshaft.
The second collar, which includes the auxiliary cam and weight section integrally formed therewith, comprises a single piece supported for rotation upon the first collar. In this manner, the present compression release mechanism includes only one moving part, and is therefore simplified in operation and in construction. Further, forces resulting from the contact between the auxiliary cam and the valve are transferred directly through the second collar and the first collar to the camshaft itself, increasing the operational life and durability of the compression release mechanism.
In one form thereof the present invention provides an internal combustion engine, including a camshaft including a longitudinal axis and at least one cam lobe, the cam lobe including a portion projecting beyond a base circle of the cam lobe for periodically engaging a valve, the camshaft further including an annular bearing surface disposed in eccentric relation with respect to the camshaft longitudinal axis; and a compression release mechanism, including a compression release member including an auxiliary cam, the compression release member supported for rotation on the annular bearing surface between a first position corresponding to engine cranking speeds in which the auxiliary cam is positioned outside of the cam lobe base circle to engage and at least partially open the valve, and a second position corresponding to engine running speeds in which the auxiliary cam is positioned within the cam lobe base circle and does not engage the valve.
In another form thereof, the present invention provides an internal combustion engine, including a camshaft having at least one cam lobe, the cam lobe including a portion projecting beyond a base circle of the cam lobe for periodically engaging a valve; and a compression release mechanism, including a first collar rotationally fixed with respect to the camshaft, the first collar having a hub portion eccentric to the camshaft; a second collar including an auxiliary cam, the second collar supported for rotation on the hub portion of the first collar between a first position corresponding to an engine cranking speed, in which the auxiliary cam is positioned outside of the cam lobe base circle to engage and at least partially open the valve, and a second position corresponding to an engine running speed, in which the auxiliary cam is positioned within the cam lobe base circle and does not engage the valve.
In a further form thereof, the present invention provides an internal combustion engine, including a camshaft having a longitudinal axis and at least one cam lobe, the cam lobe including a portion which projects beyond a base circle of the cam lobe for periodically engaging a valve, the camshaft further including an annular bearing surface having a central axis offset from the longitudinal axis; and a compression release mechanism, including a collar supported for rotation on the bearing surface, the collar including an auxiliary cam, and a weight section disposed around a portion of a circumference of the collar; a spring connecting the camshaft and the collar and biasing the collar to a first position in which the auxiliary cam is positioned outside of the cam lobe base circle to engage and at least partially open the valve, whereby at engine running speeds, the inertia of the weight section overcomes the bias of the spring, causing the collar to rotate to a second position in which the auxiliary cam is positioned within the cam lobe base circle and does not engage the valve.
In a further form thereof, the present invention provides an internal combustion engine, including a camshaft having a longitudinal axis and at least one cam lobe, the cam lobe including a portion projecting beyond a base circle of the cam lobe for periodically engaging a valve; and a compression release mechanism, including an annular bearing surface having a central axis offset from the longitudinal axis of the camshaft; and means, rotatably supported on the bearing surface, for engaging the valve at engine cranking speeds and not engaging the valve at engine running speeds.